Actually, yes, I think it's your age. But that doesn't mean you can't be friendly with these people. DD's BFF's mum was also a very young mum, while I'm an older mum (comparatively). The girls think it's hilarious that DH is old enough to be BFF's grandfather (and yes, her actual grandfather is only a few years older than him!). We all get on just fine and we've been out for drinks a few times etc or we'll hang and have a cup of tea at the start or end of play dates etc, and I like her very much and we definitely have a helpful shared/friendly relationship.... but, the age gap means that no, we haven't become super close. We're just in very very different places in our lives, notwithstanding having similar aged children.
She's just had another baby... I'm on the final slide to full blown menopause.
She's had 2 serious relationships (and a coupel of short term ones) in the years I've known her.... 've been married to DH the entire time. And she's still learning about relationships and who she is.
She's just starting her career and is in the early stages - I'm in senior leadership.
I like her a lot, we get on well, I think she feels the same about me, but no, we're not friends in the true sense of the word.
Having said that, you could at the very least develop the relationship I have with this mum and doing that is things like inviting her in for a drink or a tea before/after playdates, or suggesting a group activity with the kids, or setting up an evening and inviting others to attend. We've had some fun days out with the kids, plenty of glasses of wine or cups of tea at playdates etc.